1. Field of the Invention
One or more embodiments of the invention concerns the field of medical technology and relates to a device for shielding an implant in the body of a person during a magnetic resonance imaging, and a method for operating a magnetic resonance scanner.
2. Description of the Related Art
During a nuclear or magnetic resonance imaging (MRI=Magnetic Resonance Imaging), atomic nuclei are forced by a strong static magnetic field (B0 field) to perform a precision movement about the field direction and are excited into resonance during an excitation phase by high-frequency electromagnetic field pulses. After switching off the high-frequency pulses, the atomic nuclei go back again to their initial direction, wherein the decay times (relaxation times) are characteristic for different types of tissue. Simultaneously with the high-frequency pulses, fast switched magnetic gradient fields (B1 field) are used through which it can be achieved that the magnetic field acting on the atomic nuclei varies, for example linearly, along a predetermined direction. This causes a change of the resonance frequency which allows a spatial allocation of individual MRI signals.
Today, MRI examinations are part of the daily routine in radiological diagnostics, wherein under normal examination conditions, except for occasional discomfort, there is no risk for the patients. However, more and more patients have implants consisting completely or partially of a metallic or conductive material which could cause problems during the examination or, in the worst case, could even injure the patients. The used material and the geometry of the implant as well as the position of the latter with respect to the MRI field pulses play a significant role here. Thus, some implants act as antenna for the irradiated MRI field pulses and heat up considerably due to the generated induction currents. Subsequently, thermal damage to the surrounding tissue can occur. In electronic implants, for example cardiac pacemakers, a malfunction, for example an incorrect sensing or an undesired restart with preset start parameters (“power-on reset”) can be triggered.
For this reason, the diagnostic benefit of an MRI examination has to be carefully weighed against the risk of disadvantageous side effects. An additional difficulty is that the heating of some implants depends on the specific scan parameters and in particular on the positioning of the implants relative to the MRI field pulses so that in clinical practice very often no reliable prognoses about the expected consequences can be made. The result of all this is that persons with an implant are usually excluded from the normally gentle examination by means of MRI.
Recently, considerable efforts have been made to solve this problem. For example, new cardiac pacemaker systems were developed which are provided with specially designed pacemaker electrodes to counteract a heating by means of alternating fields. Likewise, specially designed cardiac pacemakers approved for an examination in a magnetic resonance scanner are commercially available. The disadvantage here is that these solutions involve additional costs and in some cases an impairment of the implant properties. Moreover, patients having implants not specifically suited for MRI examination cannot be examined by means of MRI.